failover: Fix error return code in net_failover_create

if failover_register failed, 'err' code should be set correctly

Fixes: cfc80d9a ("net: Introduce net_failover driver")
Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>